The present invention relates to an image processing apparatus for quantizing multi-value image data to n-value image data and, more particularly, to an image processing apparatus for quantizing multi-value image data to binary or ternary data.
As a pseudo-halftone processing system used in, e.g., a digital copying machine, a facsimile apparatus, or the like, an image processing apparatus disclosed in, e.g., "Japanese Patent Laid-Open No. 2-210959" has been is proposed.
In this image processing apparatus, an operation for binary-converting a pixel of interest to black or white using binary data of already binary-converted pixels near the pixel of interest, and adding an error generated upon binary conversion to multi-value data of non-binary-converted pixels near the pixel of interest is sequentially repeated in units of pixels.
More specifically, an average density is calculated using only already binary-converted data, and input multi-value data are binary-converted using the average density as a threshold value. For this reason, in this binary conversion method, binary conversion can be realized with a relatively small processing amount, and an error between input multi-value data and the average density, which error is generated upon binary conversion of the input multi-value data, is corrected. Thus, an image having good gradation characteristics can be obtained.
However, in the above-mentioned binary conversion method, since a pixel of interest is binary-converted according to conditions of neighboring pixels, dots are inevitably connected in a highlight portion, and the image quality in the highlight portion undesirably deteriorates.
Also, image quality in a low-density portion deteriorates.
Since an error generated upon binary conversion is added to surrounding non-binary-converted pixels, a memory for holding error data is required. Conventional method of binary required, resulting in high memory cost.
Furthermore, a conventional method of binary-converting a halftone image include a dot method, a dither method, an error diffusion method, and the like. Recently, even in the field of ink-jet recording, an output apparatus capable of outputting a plurality of kinds of values (n-value), e.g., ternary data using two kinds of, i.e., dark and light inks, or a plurality of kinds of inks, or by changing the dot size, has been developed. Thus, the binary conversion method is expanded to cope with n-value data, e.g., ternary data.
However, when quantization is performed using these methods, a pseudo-edge is undesirably generated in a portion, corresponding to a moderate change in gradation, of an image.